Automatic firearm with gas actuated breech mechanism

ABSTRACT

The actuating part of the breech mechanism comprises at least one longitudinal housing open towards the front and laterally towards the inside and towards the outside of the weapon, in which is mounted on actuating slider free to be displaced towards the front with respect to said actuating part but in rearward abutment by thrust of a rear support surface against the bottom of said housing. The actuating slider comprises, forwardly, a front stop surface projecting forwardly at the periphery of the actuating part and occurring on the withdrawal path of the corresponding movable actuating unit and, at its side oriented towards the inside of the weapon, a protuberance projecting in the direction of the breech block through the inner lateral opening of the housing of the slider concerned. This protuberance comprises a rear engagement notch adapted to come into contact with a rear shoulder fast to the breech block and situated (assuming the slider in the extreme forward position) behind said engagement notch at a distance from the latter at least equal to the withdrawal stroke which the actuating part of the breech mechanism must traverse to free the bolt from the breech block.

United States Patent [1 Pierre et al.

[ Mar. 25, 1975 AUTOMATIC FIREARM WITH GAS ACTUATED BREECH MECHANISM [75] Inventors: Bernard Pierre, Lyon; Lucien Segas,

Rueil; Georges Simon, Saint-Germain-du-Puy; Rene Volle, Bourges, all of France [73] Assignee: ETAT Francais, Paris, France [22] Filed: Oct. 9, 1973 [2l] Appl. No.: 404,199

[30] Foreign Application Priority Data Oct. I0. 1972 France 72.35764 [52] U.S. Cl. 89/190, 89/1 K [5 1] Int. Cl. F41d 5/04 [58] Field of Search 89/168, 176, 190

[ 56] References Cited UNITED STATES PATENTS 150L069 3/1950 Maillard 89/190 2536.99? l/l95l Lippert et al. 89/190 Primary ExaminerStephen C. Bentley Attorney. Agent, or FirmLarson, Taylor & Hinds [57] ABSTRACT The actuating part of the breech mechanism comprises at least one longitudinal housing open towards the front and laterally towards the inside and towards the outside of the Weapon, in which is mounted on actuating slider free to be displaced towards the front with respect to said actuating part but in rearward abutment by thrust of a rear support surface against the bottom of said housing. The actuating slider comprises, forwardly, a front stop surface projecting forwardly at the periphery of the actuating part and occurring on the withdrawal path of the corresponding movable actuating unit and, at its side oriented towards the inside of the weapon, a protuberance projecting in the direction of the breech block through the inner lateral opening of the housing of the slider concerned. This protuberance comprises a rear engagement notch adapted to come into contact with a rear shoulder fast to the breech block and situated (assuming the slider in the extreme forward position) behind said engagement notch at a distance from the latter at least equal to the withdrawal stroke which the actuating part of the breech mechanism must traverse to free the bolt from the breech block.

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AUTOMATIC FIREARM WITH GAS ACTUATED BREECH MECHANISM The invention relates to automatic firearms (especially to those of small or medium calibre, that is to say of a calibre comprised between and 50 mm) whose breech mechanism is actuated by gases in the barrel of the firearm on each shot fired and led to at least one gas-actuated mechanism comprising a movable unit adapted to coact with the abovesaid breech mechanism.

There are already known, especially through French patent No. l,602,862, automatic firearms of this type whose breech mechanism comprises, among other elements,

a breech block, with alternating longitudinal move ment, provisionally immobilized in closed position by two retractable lateral bolts, supported on inclined shoulders rigidly fixed to the breech casing of the firearm,

an actuating part of this breech block, also with alternating longitudinal movement, partially enveloping said breech block and adapted to undergo relative displacements longitudinal with respect to the abovesaid breech block, the abovesaid actuating part ensuring, at the end of its forward travel, the provisional blocking of the abovesaid lateral bolts in active position,

and two gas-actuated mechanisms, lateral to the longitudinally sliding movable unit, operating by the gas in the barrel of the firearm, the movable units of the abovesaid mechanisms causing, on their withdrawal stroke, in a first time, a retraction of the actuating part with respect to the breech block and the freeing of the lateral bolts in order to withdraw them by the reaction generated on their inclned support shoulder, then, in a second time and before the actuating part has accomplished the whole of its possible withdrawal stroke with respect to the breech block, the withdrawal of said breech block which will then be engaged and struck by the abovesaid actuating part only subsequently and with a reduced impact blow due to the factthat two parts concerned (breech block and actuating part) are both actuated by a withdrawal movement at the moment when the abovesaid blow occurs.

Although such a breech mechanism operates in satisfactory manner as a whole, it has however certain drawbacks which it is desirable to eliminate or at least to reduce.

A first drawback results from the fact that the breech block is actuated directly by the movable units of the lateral gas-actuated mechanisms, which obliges, given that these units are relatively spaced from the longitudinal plane of symmetry of the firearm, enlargement of the size of said breech block in the transverse direction, so that it comprises a bearing surface occurring effectively on the withdrawal path of the abovesaid movable units, such an enlargement of the breech block being manifested by an increased block bulk and increase in its weight and, consequently, of the inertial forces brought into play at times ofa longitudinal acceleration and deceleration of the abovesaid breech block.

A second drawback is due to the fact that the with- 6 drawal of the lateral bolts is generated by the reaction of their inclined support shoulders which act with camlike action, such behaviour, which brings into play the coefficient of friction of the bolts on their inclined support shoulders, being a function of the state of lubrication of contact surfaces of said bolts and of said shoulders, which state of lubrication varies widely with external conditions, the duration of firing, the frequency and the quality of maintenance operations, etc.

The invention comprises a certain number of features enabling the remedy to a large extent, evenly entirely, of the drawbacks mentioned above, in certain automatic firearms with breech block and with a gasoperated actuating part.

According to one of these features, intended to over come at least the first drawback mentioned in the foregoing, an automatic firearm with a gas-acutated'breech mechanism, comprises among other elements, a breech block with alternating longitudinal movement urged toward its closed position by at least one return spring, at least one retractable bolt adapted to temporarily immobilize this breech block in closed position, an actuating part with alternating longitudinal movement partially enveloping said breech block and possessing with respect to the latter, a degree of longitudinal sliding freedom enabling said actuating part (when it is in extreme forward position) to block the abovesaid retractable bolt in active position, and at least one gasactuated mechanism operating from the gas in the barrel of the firearm and comprising a movable actuating mechanism sliding longitudinally capable of causing during its withdrawal stroke, in a first time. the with drawal of the abovesaid actuating part with respect to the breech block and the freeing of the retractable bolt then, in a second time occurring after the retraction of said bolt and before the actuating part has accomplished the whole of its possible withdrawal stroke with respect to the breech block, the withdrawal of said breech block, the above said automatic firearm being characterized in that the actuating part of its breech mechanism comprises at least one longitudinal housing, open forwardly and laterally towards the inside and towards the outside of the firearm, in which is mounted an actuating slider free to be moved forward with respect to said actuating part but stopped rearwardly, by thrust of a rear support surface against the bottom of the abovesaid housing, the abovesaid actuating slider comprising,

forwardly, a front stop surface projecting forwardly to the periphery of the actuating part and occurring on the withdrawal path of the corresponding movable actuating unit, and, on its side oriented toward the inside of the firearm, a protuberance projecting in the direction of the breech block through the inner lateral opening of the housing of the slider concerned, said protuberance comprising a rear engagement notch adapted to come into contact with a rear shoulder rigidly fixed to the breech block and located (assuming the slider to be in its extreme forward position) behind said engagement notch at a distance from the latter at least equal to the withdrawal stroke which the actuating part of the breech mechanism must travel to free the bolt of the breech block. Such a feature has, with respect to the prior solution 5 consisting of making the corresponding movable actu- In fact, in the case of the prior solution, the breech block must comprise a transverse protuberance occurring on the withdrawal path of the corresponding movable actuating unit, which lead to adopting for the breech block a maximum transverse dimension corresponding substantially to that of the actuating part partially enveloping said breech block, whilst, due to the feature of the invention defined above, the withdrawal of the breech block is caused by the coming into contact of two elements (rear engagement notch of the slider and shoulder of the breech block) much closer to the axis of the breech block than the peripheral zone in which the movable actuating unit operates.

In a preferred embodiment of this first feature of the invention, the breech block and the manipulating part are actuated by two gas-actuated mechanisms and each possessing their own movable actuating unit. these two gas-actuated mechanisms being arranged symmetrically on both sides of the longitudinal plane of symmetry of the firearm, which then leads to causing the actuating part to bear two actuating sliders also arranged in symmetrical manner with respect to the abovesaid plane of symmetry.

Preferably, the breech block is held temporarily locked by two lateral bolts arranged symmetrically and pivoted towards the front around an axis perpendicular with respect to the axis of the barrel.

There can then, advantageously. be given to each slider the shape of a flat rule having, forwardly. a rectangular portion, and, rearwardly. and on the inside, a trapezoidal protuberance having its small base situated on the inside of the firearm and its rearside cut so as to form the engagement notch which has to cooperate by a abutment with the corresponding shoulder of the breech block.

When the firearm comprises two movable actuating units on both sides and above the axis of the breech block, the housings of the actuating sliders could advantageously be inclined from above to below from their outer side to their inner side, so that the sliders extend, in the transverse direction, from the level of the abovesaid movable units to the average level (lower than the foregoing one) of the abovesaid breech block.

According to another feature of the invention, capable of application independently of the preceding one, and which is intended to overcome the second drawback mentioned previously, an automatic firearm with gas-actuated breech mechanism. comprises, among other elements. a breech block with alternating longitudinal movements, at least one retractable bolt adapted to temporarily immobilize this breech block in closed position, at least one actuating part with alternating longitudinal movement capable. in extreme forward positon, of blocking the retractable bolt in active position, and at least one gas-actuated mechanism operated by the gas in the barrel of the firearm and comprising a movable actuating unit sliding longitudinally and capable of causing the withdrawal of the abovesaid actuating part, then ofthe abovesaid breech block, once the latter is unlocked, the abovesaid automatic firearm being characterized in that it comprises also mechanical transmission means inserted between the above said retractable bolt and the abovesaid movable actuating mechanism and arranged so that, on the initial phase of its withdrawal stroke, the mobile unit peremptorily causes the retraction of the bolt.

Such mechanical transmission means can advantageously be constituted by a simple pusher of which one end is in contact with the bolt to be actuated and of which the other end projects into the housing of the movable actuating unit thus occurring on the with drawal path of a shoulder belonging to the abovesaid movable unit, the orientation of the abovesaid pusher being such that its depression, caused by the abovesaid shoulder, peromptorilly obliges the bolt to pass from its active position to its retracted position.

In any case and whatever the embodiment adopted, each gas-actuated mechanism with a mobile unit could advantageously be connected to a source of gas under pressure independent of the weapon proper (reservoir of compressed air or pyrotechnic gas generator for example) so that said gas-actuated mechanism can be actuated at will, independently offiring, to cause a rearming operation, the weapon being assumed at firing arrest with a closed breech, for example as a result of a misfiring.

In this case. the abovesaid movable unit can advantageously be made to include an intermediate hollow piston bounding, with a plunger entering said hollow piston, and belonging to another piston situated on the side of the breech mechanism to be actuated, a variable volume chamber in which opens an inlet for gas under external pressure. the abovesaid hollow piston being stopped forwardly, whilst the abovesaid plunger has a length corresponding to the release stroke of the one or more bolts so that, on the operation of rearming and once the breech block is unlocked, the abovesaid piston equipped with a plunger offers an increased surface to the effect of the external gases in the course of expansion. which is obviously advantageous, given that these gases, partially expanded, must then cause the recoil of the whole movable breech mechanism against the effect of the one or more return springs, as well as the extraction of the unfired cartridge.

To illustrate more accurately still the various features of the invention, there will now be described a preferred embodiment, but which is in no way limiting, with reference to the accompanying drawings in which FIG. I shows, in horizontal section, with parts cut away, the breech mechanism of an automatic firearm according to the invention. this mechanism being shown in the position of locking of the breech block.

FIG. 2 is a view from below of the locking device of the breech block of said firearm,

FIG. 3 is a transverse section along the line III-Ill of FIG. 1.

FIG. 4 shows, in a horizontal half-section, the abovesaid breech mechanism in the situation where the breech block has just become unlocked,

FIGS. 5 to 8, lastly, are diagrammatic plan views with horizontal half-section of the breech mechanism concerned, said views illustrating characteristic successive phases of the operation of said mechanism.

The automatic firearm illustrated in FIGS. 1 to 4 is an automatic cannon of 20 mm calibre whose breech mecanism, actuated by gas in the barrel of the firearm, is of the open breech type, that is to say the type, quite conventional, for which the breech block of the firearm is held, during interruption of firing. in a withdrawn position from which it is returned fowardly by at least one return spring on resumption of firing, the abovesaid breech block being temporarily locked when it is in closed position.

The breech mecanism of this automatic cannon comprises essentially,

a breech block 1 bearing a firing device (not shown),

said breech block being mounted sliding longitudinally in a breech casing 2 and being subjected, on one hand, to the indirect action of two return springs 3 urging it forwardly, that is to say towards its closed position illustrated in FIG. 1, for which it is supported on the cartridge C in firing position, and, on the other hand, at the end of the rearward stroke, to the action of a rear damping device 4 constituted for example by a stack of elastic washers, two lateral bolts 5, retractable by pivoting around a vertical axle, each pivoted through their front end on the breech block 1, said bolts 5 being supported by their rear end, in their locking position illustrated in FIG. 1, on the bearings 6 fast to the breech casing 2, whilst, in their retracted position illustrated in FIG. 4, these bolts 5 escape from these bearings 6,

an operating part 7 of U-shaped transverse section, with an alternating longitudinal movement, subject to the action of return spring 3 and envelopping the breech block 1 below and laterally as seen in FIG. 3 and possessing, with respect to the latter, a degree of longitudinal sliding freedom enabling said operating part 7, when it is in its extreme forward position shown in FIG. 1, to block the bolts 5 by engagement, between said bolts and the breech block I, of flexible lateral tabs 7a provided at the front of the abovesaid operating part 7,

two lateral stops 8 (seen clearly in FIG. 2) intended for the actuation of the bolts 5 and borne by the bottom of the breech block 2, each of these stops 8 being capable of coacting with the ramp 9 borne by the corresponding bolt 5, to necessarily oblige said bolt, at the end of the forward stroke of the breech block 1, to occupy its active position (seen in FIG. 1) for which it temporarily locks the said breech block 1 with respect to the breech casing 2,

and two lateral gas-operated mechanisms operating by gas in the barrel of the firearm and each comprising a movable unit (denoted as a whole by reference 10) formed by pistons sliding longitudinally in a bore ll formed in the breech casing 2, the two movable units 10 being capable, when they are sent rearwardly by the pressure of the gases taken from the barrel of the firearm on each shot fired, of projecting inside the breech casing 2 and then acting on the operating parts 7 in the indirect manner whic will now be indicated.

In fact. instead of providing direct action of the movable units 10 on the operating part 7, as has been done hitherto, the main feature of the invention provides indirect actuation of said operating part, this indirect operation being obtained in the following manner:

There is provided, in the operating part 7, two lateral housings 12 open forwardly and laterally towards the inside and towards the outside of the firearm, these housings each receiving an actuating slider 13 free to be moved forwardly with respect to the abovesaid operating part along a longitudinal gap .1, (FIG. 4), but stopped rearwardly by thrust of a support surface S against the bottom F, of the housing 12. The outer lateral opening of said housing runs over the whole length of said housing, so as to enable an easy positioning of the corresponding slider 13.

Each actuating slider 13 has transverse sections of rectangular shape and it has the general shape of a flat rule comprising, as clearly seen in FIG. 4.

Towards the front, a rectangular zone 13a of which the front section constitutes a stop surface S projecting fowardly at the periphery of the operating part 7 and occurring on the recoil path of the corresponding movable actuating unit 10,

and, on its side oriented towards the inside of the firearm, a protuberance 13b (of generally trapezoidal shape with its small base situated on the side of the interior of the weapon) projecting in the direction of the breech block 1 through the inner lateral opening of the housing 12, said protuberance 13b comprising a rear engagementnotch 13c adapted to come into contact with a rear shoulder E fast to the breech block 1 and situated, when the slider 13 is in its extreme forward position shown in FIG. 1, behind the abovesaid engagement notch 13c at a distance D, from the latter at least equal to the recoil stroke which the operating part7 must traverse to release the bolts 5.

It should be noted that each slider 13 comprises, on its inner edge, a support notch 13d engaged in a longitudinal groove 14 formed in the breech block 1, this support notch 13d being intended to coact, at the end of locking, with a percussion mechanism mounted slidably in the breech block and diagrammatically illustrated in FIG. 4, in order to ensure then the firing of the cartridge introduced into the cartridge chamber of the firearm.

It is seen in FIG. 1, that, for such a breech mechanism, the shoulder E receiving the thrust of the sliders 13 is much closer to the vertical plane of symmetry of the firearm than the movable actuating units 10, which enables the breech block 1 to be given a maximum cross section much more reduced than that of the operating part 7 (see FIG. 3) and, in consequence, to re-' duce the bulk of this breech block and the inertial forces brought into play in said breech block whose fragility and risks of breakage are thus reduced.

Besides, each movable actuating unit 10 comprises rearwardly, a piston 10a provided with a shoulder E capable of repelling, when it arrives at its level after a recoil stroke, which has ensured the disengagement of the blocking tabs 7a from the operating part 7, a pushrod 15 whose displacement then causes the retraction of the bolt 5 situated on the side of the movable unit 10 concerned.

Lastly, and as shown in FIG. 1, the piston 10a of each movable unti 10 comprises, forwardly, a plunger 16 entering with slight fraction into a bore 17 formed in the rear end of a piston 10b, stopped forwardly, forming part of the movable actuating unit concerned and situated immediately forward of the abovesaid piston 10a.

the chamber constituted by this bore 17 then being capable of supply with gas under pressure (generated by a pyrotechnic gas-generator) through supply orifices 18 opening in the front end of the abovesaid chamber. There will now be described the operation of such a breech mechanism with reference to the diagrams of FIG. 5 to Sin which the same reference numbers or letters denote the same elements as in FIGS. 1 to 4.

FIG. 5 represents the breech mechanism at the moment of departure of the shot, with its breech block 1 held in closed position by the bolts 5, themselves locked in active position by the tabs 70 of the operating part 7.

FIG. 6 shows the same mechanism, some millisec onds later, whilst the introduction of gas into the barrel of the firearm has intervened and the pistons a have started to move the operating part 7 rearwardly, through slider 13, over a distance such that the tabs 7a of each part have released the bolts 5, the engagement notches 130 of said sliders still not having reached the shoulder E, of the breech block 1 and the shoulders E of the pistons 10a being then at the point of engaging the push rods of the bolts 5. The breech block 1 is still immobile, but, on the contrary. the operating part 7 is launched rearwardly.

FIG. 7 corresponds to a following phase of operation, for which, on one hand, the shoulders E of the pistons 10a have caused the withdrawal of the bolts 5 through the push-rods l5, and, on the other hand, the engagement notch 13c of the sliders 13 has reached the shoulder E which has the effect of launching in its turn the breech block 1 (now unlocked) rearwardly.

FIG. 8 lastly, corresponds to the following stage where the sliders 13 have abandoned the actuating pistons 10a and where the operating part 7 (launched in front of the breech block 1 has engaged the abovesaid breech block 1 with an engagement shock reduced due to the fact that the two parts concerned (breech block I and operating part 7) were both actuated by a withdrawal movement at the moment of the engagement.

It is lastly proper to note that the process of withdrawal of the movable parts is the same when a rearming operation is effected by supplying the chamber 17 by means of gases provided from a pyrotechnic charge. the unlocking of the breech block being effected by the action of the gases on the section of the plunger 16 and, once this unlocking has been effected, the withdrawal of the breech block being carried out by the action of the partially expanded gases on the complete section (greater than that of the plunger 16) of the head of the actuating piston 100.

As is self-evident and as emerges already from the foregoing, the invention is in no way limited to those of its types of applications and embodiments which have been more especially envisaged; it encompasses, on the contrary, all modifications.

We claim:

1. Automatic firearm with a gas-operated breech mechanism and comprising: a breech block with alternating longitudinal movement, urged towards its closed position by at least one return spring; at least one retractable bolt for temporarily immobilizing this breech block in its closed position; an actuating part with alternating longitudinal motion, partially enveloping said breech block and possessing, with respect to the latter, a degree of longitudinal sliding freedom enabling said actuating part, when it is an extreme forward position, to block said retractable bolt in active position; and at least one gas-actuated mechanism in the barrel of the firearm and comprising a movable actuating unit sliding longitudinally and capable of causing, in the course of retraction, at a first phase, the withdrawal of said actuating part with respect to the breech block and the release of the retractable bolt, then, in a second phase occurring after the withdrawal of said boot and before the actuating part has completed its possible withdrawal stroke with respect to the breech block, withdrawal of said breech block, said actuating part of the breech mechanism comprising at least one longitudinal housing open towards the front and laterally towards the inside and towards the outside of the firearm, in which is mounted an actuating slider free to be displaced towards the front with respect to said actuating part but in rearward abutment by thrust of a rear support surface against the bottom of said housing, said actuating slider comprising,

forwardly, a front stop surface projecting forwardly at the periphery of an actuating part and occurring on the withdrawal path of the corresponding movable actuating unit,

and, at its side oriented towards the inside of the firearm, a protuberance projecting in the direction, of the breech block through the inner lateral opening of the housing of the slider concerned, said protuberance comprising a rear engagement notch adapted to come into contact with a rear shoulder fast to the breech block and situated, when the slider is in the extreme forward position, behind said engagement notch at a distance from the latter at least equal to the withdrawal stroke which the actuating part of the breech mechanism must traverse to free the bolt from the breech block.

2. Automatic firearm according to claim 1, whose breech block and the actuating part are actuated by two gas-actuated mechanisms and each possessing their own movable actuating unit; these two mechanisms being arranged symmetrically on both sides of the low gitudinal plane of symmetry of the firearm. wherein the actuating part bears two actuating sliders arranged symmetrically with respect to said plane of symmetry.

3. Automatic firearm according to claim 1, wherein the breech mechanism comprises two lateral bolts arranged symmetrically and pivoted towards the front around axis at right angles with respect to the barrel axis.

4. Automatic firearm according to claim 1, wherein each slider has the shape of a flat strip having, towards the front, a rectangular portion and, towards the rear and on the inside, a trapezoidal protuberance having its small base situated on the inside of the firearm and its rear side shaped so as to form the engagement notch which has to cooperate by abutment with the corresponding shoulder of the breech block.

5. Automatic firearm according to claim 4, wherein each slider comprises, on its inner edge, a support notch engaged in a longitudinal groove formed in the breech block, this support notch cooperating, at the end of locking, with a percussion mechanism mounted slidably in the breech block, for ensuring the percussion of the cartridge introduced into the cartridge chamber of the firearm.

6. Automatic firearm according to claim 1, comprising two movable actuating units, situated on both sides and above the axis of the breech block, wherein the housings of the actuating sliders are inclined from above to below from their outer side to their inner side, so that the sliders extend, in the transverse direction, from the level of said movable actuating units to the middle level, below the preceding one, of said breech block.

7. Automatic firearm according to claim 1, wherein each gas-actuated mechanism with a movable actuating unit is connected to a source of gas under pressure independent of the firearm proper so that said gaspiston being in abutment forwardly, whilst said piston rod has a length corresponding to the free travel of the one or more bolts so that; once the breech block is unlocked, said other piston equiped with the piston rod offers an increased surface to the effect of the external gases in the course of expansion.

9. Automatic firearm according to claim 1, wherein said rear shoulder is situated behind said engagement notch at aa distance greater than said withdrawal stroke. 

1. Automatic firearm with a gas-operated breech mechanism and comprising: a breech block with alternating longitudinal movement, urged towards its closed position by at least one return spring; at least one retractable bolt for temporarily immobilizing this breech block in its closed position; an actuating part with alternating longitudinal motion, partially enveloping said breech block and possessing, with respect to the latter, a degree of longitudinal sliding freedom enabling said actuating part, when it is an extreme forward position, to block said retractable bolt in active position; and at least one gasactuated mechanism in the barrel of the firearm and comprising a movable actuating unit sliding longitudinally and capable of causing, in the course of retraction, at a first phase, the withdrawal of said actuating part with respect to the breech block and the release of the retractable bolt, then, in a second phase occurring after the withdrawal of said boot and before the actuating part has completed its possible withdrawal stroke with respect to the breech block, withdrawal of said breech block, said actuating part of the breech mechanism comprising at least one longitudinal housing open towards the front and laterally towards the inside and towards the outside of the firearm, in which is mounted an actuating slider free to be displaced towards the front with respect to said actuating part but in rearward abutment by thrust of a rear support surface against the bottom of said housing, said actuating slider comprising, forwardly, a front stop surface projecting forwardly at the periphery of an actuating part and occurring on the withdrawal path of the corresponding movable actuating unit, and, at its side oriented towards the inside of the firearm, a protuberance projecting in the direction of the breech block through the inner lateral opening of the housing of the slider concerned, said protuberance comprising a rear engagement notch adapted to come into contact with a rear shoulder fast to the breech block and situated, when the slider is in the extreme forward position, behind said engagement notch at a distance from the latter at least equal to the withdrawal stroke which the actuating part of the breech mechanism must traverse to free the bolt from the breech block.
 2. Automatic firearm according to claim 1, whose breech block and the actuating part are actuated by two gas-actuated mechanisms and each possessing their own movable actuating unit; these two mechanisms being arranged symmetrically on both sides of the longitudinal plane of symmetry of the firearm, wherein the actuating part bears two actuating sliders arranged symmetrically with respect to said plane of symmetry.
 3. Automatic firearm according to claim 1, wherein the breech mechanism comprises two lateral bolts arranged symmetrically and pivoted towards the front around axis at right angles with respect to the barrel axis.
 4. Automatic firearm according to claim 1, wherein each slider has the shape of a flat strip having, towards the front, a rectangular portion and, towards the rear and on the inside, a trapezoidal protuberance having its small base situated on the inside of the firearm and its rear side shaped so as to form the engagement notch which has to cooperate by abutment with the corresponding shoulder of the breech block.
 5. Automatic firearm according to claim 4, wherein each slider comprises, on its inner edge, a support notch engaged in a longitudinal groove formed in the breech block, this support notch cooperating, at the end of locking, with A percussion mechanism mounted slidably in the breech block, for ensuring the percussion of the cartridge introduced into the cartridge chamber of the firearm.
 6. Automatic firearm according to claim 1, comprising two movable actuating units, situated on both sides and above the axis of the breech block, wherein the housings of the actuating sliders are inclined from above to below from their outer side to their inner side, so that the sliders extend, in the transverse direction, from the level of said movable actuating units to the middle level, below the preceding one, of said breech block.
 7. Automatic firearm according to claim 1, wherein each gas-actuated mechanism with a movable actuating unit is connected to a source of gas under pressure independent of the firearm proper so that said gas-actuated mechanism can be actuated at will, independently of firing, to cause a rearing action, the firearm being assumed inactive with a closed breech block, for example as result of a mis-fire.
 8. Automatic firearm according to claim 7, wherein each movable actuating unit comprises a hollow intermediate piston bounding, with a piston rod stemming into said hollow piston and belonging to an other piston situated on the side of the breech mechanism to be actuated, a chamber of variable volume which an injection of gas under external pressure enters, said hollow piston being in abutment forwardly, whilst said piston rod has a length corresponding to the free travel of the one or more bolts so that; once the breech block is unlocked, said other piston equiped with the piston rod offers an increased surface to the effect of the external gases in the course of expansion.
 9. Automatic firearm according to claim 1, wherein said rear shoulder is situated behind said engagement notch at aa distance greater than said withdrawal stroke. 